Does Deep Borehole Disposal of HLRW has a …...atw Vol. 62 (2017) | Issue 1 ı January Decommissioning and Waste Management Does Deep Borehole Disposal of HLRW has a Chance in Germany?

atw Vol. 62 (2017) | Issue 1 ı January

Decommissioning and Waste ManagementDoes Deep Borehole Disposal of HLRW has a Chance in Germany? ı Guido Bracke, Frank Charlier, Axel Liebscher, Frank Schilling and Thomas Röckel

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Does Deep Borehole Disposal of HLRW has a Chance in Germany?Guido Bracke, Frank Charlier, Axel Liebscher, Frank Schilling and Thomas Röckel

1 Introduction The disposal of high-level radioactive waste (HLRW) using deep boreholes in geological formations (salt rock) previously has been considered in Germany a long time ago [GOM 82] but was not pursued then. Furthermore, safety requirements [BMU 10] were identified for a geological underground mine.

Deep borehole disposal (DBD) may offer some advantages such as a better containment due to greater depth, faster disposal and lower costs. It is under discussion currently by the Department of Energy in the USA [NWTRB 16], in the UK [GIB 14] and in Germany [BRA 15].

Therefore, the German Commis-sion “Storage of high-level radioactive waste”, which was active from 2014 to 2016 [KOM 16], discussed deep borehole disposal as an alternative dis posal option and requested a study addressing several questions [BRA 16].

In the following, an overview is presented of the concept of HLRW disposal in deep boreholes based on results of [BRA 16]. Furthermore, the DBD concept is discussed with regard to its compatibility with recommen-dations of the Commission and the safety requirements of the BMUB [BMU 10].

2 Waste volume in Germany

The volume of high-level radioactive waste is limited in Germany due to the phase-out from nuclear energy. The waste forms are mainly spent fuel elements from power reactors ( approx. 35,000 pieces), cans with vitrified waste from reprocessing ( approx. 8,000 pieces) and some spent fuel elements from research reactors (approx. 2,000 m3) [see e. g. KOM 16].

3 Concept for deep borehole disposal

A concept for deep borehole disposal (DBD) should show containment of the high-level radioactive waste in the long-term from the biosphere (subject of protection). Technical feasibility and operational safety have to be demonstrated.

3.1 Safety requirements and the disposal concept

The great depth which can be reached by boreholes can contribute to a safe containment if overlying sedimentary rocks provide additional geological barriers (multiple geological barrier concept). The large distance of the disposed waste from the surface is also expected to ensure long migration times for radionuclides released from the waste form to human beings and the subject of protection (e.g. ground-water, biosphere). The expected time period for keeping a borehole open for waste emplacement is considerably shorter than for a mined geological repository, so proliferation risks can be assumed to be much lower.

The general requirements for the presented generic concept for DBD are:

� The disposal of vitrified waste containers and spent fuel should be technically feasible. Other waste is not considered here.

� The concept should provide a multi-barrier concept.

� The retrieval of waste should be possible.

� The concept should also allow monitoring during the operational and post-closure phases

� The appropriate lithology should be available in Germany and char-acterisable.

The use of multiple, independent geo-logical barriers formed by e.g. clay and salt layers together with seals, provide the main safety functions of the generic concept. This means that boreholes have to be sealed effectively within these barriers to restore the functionality of the barriers. Further-more, only very slow groundwater movement should be probable at great depths which ideally restricts radio-nuclide migration to diffusion alone.

The generalized concept foresees disposal in the geological bed-rock (which is most likely a crystalline rock) which should be overlain by at least two redundant or diverse geo-logical barriers. Ideally, an additional

geological features can act as gas trap below these barriers. The minimum depth for DBD is set at 1,500 m with a maximum depth of 3,500 m. This will facilitate location of sites with several independent geological barriers and exclude with greater certainty glacial impacts on barriers and waste.

The maximum depth should be optimized by assessing state-of-the-art drilling, disposal technology and the outcome of safety analyses. A vertical borehole this generic concept is preferred over inclined boreholes but multiple and deviating boreholes are possible [DEA 15, TIS 06]. The concept is depicted schematically in Figure 1.

Possible geological barriers over-lying the disposal zone (designated zone) are:

� Clay rock: bedded clay which can ensure retardation and contain-ment. Fig. 1 shows an alternating sequence of clay and sandstone.

� Salt rock: bedded salt with high sealing capacity and self-sealing ability based on its visco-plastic characteristics. Fig. 1 shows bedded and domal salt.

These barriers should be combined. At least two independent barriers should be available.

A further possible feature would be porous rock (e.g. sandstone) acting as a trap for gases which could be released from the disposal zone. Fig. 1 shows a sandstone formation below the salt layer acting as a gas trap. Such settings are naturally occuring in Germany and can be found undisturbed.

3.2 ExplorationGeological exploration is necessary to find and characterize the site. This can be performed using a number of standard technologies which also include exploration boreholes with logging and coring. Exploration bore-holes may be used at later stages for monitoring or can be developed as boreholes for disposal.



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3.3 Container and casingContainers have to withstand the geomechanical and geochemical conditions experienced during dis-posal operations. If recovery of con-tainers is foreseen for a certain period of time beyond disposal operations, the container should also be able to withstand conditions in the disposal zone. The latter is a design require-ment for the canisters which may be achieved by wall thickness allowance or by material selection.

A Deep Borehole Container – Retrievable (DBC-R) was designed using austenitic steel (Figure 2). The size of the container was derived from the diameter of the canisters with vitrified waste (0.435 m), which are not pressure resistant due to a head space, and the length of the fuel rods from spent fuel elements (approx. 4.5 m) assuming that one container should suit all waste types. The length of the container is about 5.6 m. The container can fit three canisters or an assembly of rods from spent fuel elements.

The thickness of the of the DBC-R (and therefore the diameter) in-creases with vertical stacking and disposal depth. (Table 1). The diameter of the DBC-R is a minimum of about 55 cm for disposal from 3,000 m to 3,600 m depth with allowance for temperature and some corrosion.

The total outer diameter of the casing is 70 cm considering a wall thickness of approx. 6 cm and play. Therefore a borehole diameter of 75 cm including some allowance for uncertainties could suffice for 3,600 m depth (Figure 3). The depth of 5,000 m was included at the request of the Commission [BRA 16].

Further optimization of the canis-ter’s design and material is expected.

By varying the thickness or material of the wall, corrosion or other degra-dation mechanisms can be countered.

4 Boreholes and drilling technology

Technical feasibility and costs of drilling are important factors for disposal in geological formations. Classical drilling technologies are used in conventional and unconventional oil and gas production, in geothermics and mining. Experience is also available from experimental drilling and research.

The diameter of boreholes ranges from cm to m (shaft sinking) in drilling technology. Since DBD of waste containers with diameters of 55 cm and more is envisaged, the borehole diameter must be large (Tab. 1). Physical constraints have to

be considered, as boreholes become wider and deeper.

The difference between the pres-sure of overburden of the rock and the pressure inside the boreholes increases with depth. The differential stresses can be so high that boreholes may collapse. This differential stress can be lowered if the boreholes are filled with a fluid. The hydrostatic pressure of the fluid reduces the effec-tive rock pressure on the walls of the borehole or the casing.

Thus, geomechanical stability is a limiting factor for the diameter and the depth of boreholes. This is relevant during site selection. The in- situ stress field has to be observed when setting the orientation of the borehole and avoiding the failure of the wall of the borehole.

| Fig. 1. Concept for borehole disposal in deep geological formations (example).

| Fig. 2. Deep Borehole Container – Retrievable (DBC-R) with cans and rods (rod is not to scale).

| Fig. 3. Section through a borehole with casing and DBC-R.

| Tab. 1. Maximum depth, wall thickness of DBC-R, number of boreholes for German HLRW waste (assuming disposal between 3 000 m and maximum depth) and approx. diameter of borehole (assuming a wall thickness of 6 cm for the casing).

Max. depth of borehole

Wall thickness of DBC-R

DBC-R per borehole

Number of boreholes

Approx. diameter of borehole

3,600 m 4.5 cm 103 107 75 cm

4,200 m 6.5 cm 205 55 80 cm

5,000 m 10 cm 363 31 90 cm



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Using drilling fluids (drill mud), boreholes can be drilled safely to great depths as shown by the status of technology and experience from more than 100,000 boreholes for oil and gas in different geological formations. Larger diameters were realized in research drillings (see e.g. [ENG 96]) than in the oil and gas industry. Segmental logging of boreholes gives information on lithology, porosity, conductivity, density of the formation, and rock alterations in the vicinity of the wall of each borehole.

Most drilling technologies use a drilling fluid which facilitates drilling and cleaning of the borehole. In addition to stabilization with drilling fluid, boreholes are finished with several steel casings which prevent wall collapse and inflow, and which separate different hydrological layers. The composition of this fluid may vary. After well completion, the drilling fluid is usually replaced by a borehole fluid with different charac-teristics. Cementation of the casing is usually required.

4.1 Borehole designBoreholes are cased in order to reach the desired depth for disposal safely by drilling. The detailed design of the casing set is based on subsurface data such as formation pressures, rock strengths, wellbore orientation and stress field.

The borehole drilled must be large enough to accept the casing string and to allow room for cement between the outside of the casing and the hole. A wellhead is usually installed on top of the first casing string after it has been cemented in place.

The inside diameter of the casing must be large enough that the next bit fits into it to continue drilling. Thus, each casing string has a progressively smaller diameter.

Casing design (Figure 4) for each size is performed by calculating the worst conditions that may be faced during drilling and production. Mechanical properties of designed pipes such as collapse resistance, burst pressure, and axial tensile strength must be sufficient for the worst conditions.

Casing strings are supported by casing hangers that are set in the wellhead.

The minimum diameter of a borehole with casing for disposal is approx. 75 cm assuming DBC-R- containers with canisters of vitrified waste. A safety margin against collapse and other criteria should be

provided by the casing and by the container. The experience of long-term stability and tightness of bore-hole casings covers more than 100 years. More recent experience is provided by the drillings at the KTB site [ENG 96], Groß-Schönebeck [KWI 089], Großbucholz [KWI 08], Urach [TEN 00], Soultz-sous-Forets [TIS 06], Gravberg [JUH 98] and Kola SG 3 [FUC 12], which are publicly docu-mented. These wells have reached greater depths than considered here in the concept for final disposal, but exhibit smaller diameters.

Figure 5 shows the casing set of the KTB site, which had been drilled ca. 25 years ago. The outer diameter of the borehole with 44.5 cm at 3,000 m is near to the outer diameter of approx. 75 cm given in Tab. 1 for a depth of 3,600 m. The red retangle shows the diameter of the DBC-R of 55 cm.

4.2 Drilling and wellbore logging

The most widely used method is rotary drilling. Oil well drilling utilises tri-cone roller, carbide embedded, fixed- cutter diamond, or diamond- impregnated drill bits to wear away at the cutting face. This is preferred because there is no need to return intact samples to the surface for assay as the objective is to reach a formation containing oil or natural gas. Sizable machinery is used, enabling depths of several kilometres to be penetrated. Rotating hollow drill pipes carry down bentonite and barite infused drilling muds to lubricate, cool, and clean the drilling bit, to control downhole pres-sures, to stabilize the wall of the bore-hole and to remove drill cuttings. The mud returns back to the surface around the outside of the drill pipe, called the annulus. Examining rock chips extracted from the mud is known as mud logging. Another form of well logging is electronic and is frequently

| Fig. 4. Casing (schematical).

| Fig. 5. Casing of the KTB (schematical).



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employed to evaluate the existence of possible oil and gas deposits in the borehole. This can take place while the well is being drilled, using “Measure-ment While Drilling tools”, or after drilling, by lowering measurement tools into the newly drilled hole.

Deviated boreholes currently reach 10 km distance to the wellhead and are frequently used especially in off-shore-drilling.

[ARN 11] concluded that with today’s readily available technology, drillings can be done with a diameter of 43,2 cm (17") down to 5,000 m. Research and development is neces-sary to generate drilling technology for standard larger diameters since current drilling technology in the oil and gas industry aims for diameters as small as possible to reduce costs. The expert assessment is that borehole diameters up to 75 cm at 3,500 m are feasible with enhanced technology. Borehole diameters of 100 cm at 5,000 m depth as requested by the Commission [BRA 16] were assessed not to be safely operable with current technology.

4.3 Boreholes and disposal operation

A drilling fluid and a cemented casing are necessary for drilling and opera-tional safety aspects during disposal. The typical diameter of a borehole including casing was assessed to be 75 cm for a disposal depth from 3,000 to 3,600 m and increases with depth and number of stacked containers (Tab. 1). Calculations show that a distance of 50 m between disposal boreholes should be sufficient to exclude mutual thermal and other effects.

Disposal is planned for a zone where all geological barriers are functional. This zone is called the designated zone with a safety distance to the geological barriers. The zone where all of the multiple barriers are fully functional and a containment can be provided is called the retention zone. The retention zone is character-ised by having at least one geological barrier. The transfer zone is located above the topmost geological barrier.

4.4 Operational phaseDuring disposal operations any con-tainer is removed from the biosphere and has to pass a transfer zone. Below this zone, at least one geological barrier is functioning and can provide complete containment after back-filling and sealing of the borehole. It is obvious that technical measures have

to ensure safety for the biosphere and the transfer zone, as is the case for any other disposal technology.

The complete borehole is cased and cemented. After installation of the cemented casing, the drilling fluid can be replaced by a borehole fluid specifically designed for disposal operations. Solid-free borehole fluids are used in order to allow recovery during the phase of operation for disposal. This borehole fluid should be compatible with the casing and containers to minimize corrosion, have a sufficient density to ensure borehole stability, a suitable viscosity, should inhibit corrosion and have a low complexing ability for radio-nuclides.

At least two casings are foreseen within the biosphere (subject of pro-tection). The disposal zone is equipped with a cemented casing. An additional casing (liner extension), which is not cemented, is installed for disposal and represents a safety feature within the subject of protection and transfer zone, reaching down to the retention zone, where the disposal of containers is performed. This liner extension can be removed completely with a container in case of sticking (Figure 6). A hydraulic gate allows recovery of any contaminated borehole fluid.

After installation of the cemented casing, the drilling fluid can be replaced by a borehole fluid specifi-cally designed for disposal operations. Solid-free borehole fluids are used in order to allow recovery during the

phase of operation for disposal. This borehole fluid should be compatible with the casing and containers to minimize corrosion, have a sufficient density to ensure borehole stability, a suitable viscosity, should inhibit corrosion and have a low complexing ability for radionuclides. After dis-posal, each individual canister may be cemented replacing the borehole fluid by cement to minimize fluid volumes and to separate the disposed canister from following disposal operations.

4.5 Borehole sealsAfter completion of disposal and cementation of the well within the designated zone, boreholes should be fully sealed and abandoned. Borehole seals should ensure that there is no intrusion of groundwater into the dis-posal zone and that no contaminants from the disposal zone are released. The seals will re-establish the charac-teristics of the geological barriers drilled through. Sealing of the bore-hole may be achieved in several ways with materials of proven longterm stability (salt, clay, bitumen) and over the entire length of the borehole. A fa-vourable method for sealing a bore-hole utilises the creep behaviour of salt rock /KRE 09/, which is foreseen in the concept.

Figure 7 shows a cased borehole (1.), which is reamed completely including removal of the casing in the salt formation (2.). Due to the rela-tively small diameter of the borehole, the high temperature (above 100 °C)

| Fig. 6. Borehole and disposal operation.



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and high pressure of the salt rock, the salt creeps within hours or days (3.) and the borehole is sealed effectively along its reamed length (4.). The sealing process may be enhanced and supported by filling the reamed sec-tion with crushed rock salt. Compara-ble sealing operations can also be per-formed in clay formations.

5 Safety of disposal operations

Available disposal technologies are wireline, drill string, coiled tubing, free fall (in the borehole fluid) and liner emplacement modes for instru-ments of any kind. The main differ-ences to conventional oil and gas industry are the remote, person-nel-free operation of the transfer and the huge loads in waste disposal con-tainers compared to probes. A dis-posal technology for containers in shallower boreholes has already been demonstrated successfully in princi-ple [FIL 10].

The proposed containers are not self-shielding. Therefore measures for radiation protection have to be prepared and a detailed design of such a facility for emplacement in deep boreholes should be developed to comply with radiation protection during operations. The technical con-cept for an emplacement facility should include completely encased surface facilities and the borehole with its casings. The container should be delivered in a transfer cask through a lock to the disposal facility and con-nected to the emplacement string for disposal. An additional backup wire serves as a further safety measure. When the container is connected to the string, the lock can be opened to lower the container into the well. The

descent of each container is slowed down hydraulically by the borehole fluid. Displaced borehole fluid is collected in a dedicated tub. The fluid is monitored for contaminants and radionuclides. The container is re-leased from the emplacement string when the final disposal position is reached. The emplacement string is removed and monitored for con-tamination after the disposal of each container. The facility should run in automatic mode so that no personnel are required close to the HLRW. The safe emplacement has to be demon-strated.

The design of the containers need not be self-shielding, but should be aerosol tight for a period of 500 years after emplacement. This is required by [BMU 10] with the objective to facili-tate a safe recovery. The present design of the container using steel will

inevitably lead to some corrosion due the presence of water and the high temperature in the disposal zone. Therefore, further research and development is necessary on the subject of container design for bore-hole disposal.

Provided that the borehole exists and is ready for disposal, there is no need for dedicated research on instal-lation of rigs or on transportation of loads for disposal since this is stand-ard technology. If deviating boreholes are foreseen, a high inclination should be planned to minimize friction of containers during emplacement.

6 Site selection criteria and safety assessments

[KOM 16] favours disposal in geo-logical formations using mining tech nology with retrievability and recoverability. [KOM 16] also men-tioned that disposal using boreholes in geological formations is the only realistic alternative option over transmutation or long-term interim storage. If disposal in deep boreholes is to have a chance in Germany, the concept and the selected site have to comply with German safety require-ments (currently under revision) [BMU 10] and have to undergo a site selection procedure with (prelimi-nary) safety assessments [KOM 16].

The containment providing rock zone (CPRZ) is a key element of the safety requirements [BMU 10]. The concept for borehole disposal takes advantage of multiple geological barriers. These barriers are clay or salt rock layers, which can be used to define multiple and different possible CPRZ´s of types A (enclosure of waste

| Fig. 7. Steps of borehole sealing (geology not to scale, modified after [KRE 09]).

| Fig. 8. Possible CPRZ for borehole disposal.



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by CPRZ in the host rock) or Bb (large lateral extension of CPRZ overlying the waste in the host rock) [AKE 02] (Figure 8). If disposal takes place directly in a salt or clay formation, type A is possible. Disposal including a crystalline basement should have at least a CPRZ of type Bb, if no con-tainment is provided by host rock and technical barriers [KOM 16].

A generic concept for disposal in deep boreholes is shown above. The general methodology for safety assessments – which is also discussed in [KOM 16] – should be adapted in some of the technical details (e.g. shaft seals could be seen as equivalent to borehole seals).

Any assessment will be based on the safety requirements [BMU 10] (under revision) and will make analogous use of the requirements and criteria for site selection for geological disposal in a mine given by [KOM 16]. The technically important requirements for site selection have exclusion, minimum and weighting geo-scientific criteria.

Although these requirements and criteria of [KOM 16] are intended to be applied to sites in geological forma-tions using a mined repository, they are discussed here for application to a concept using deep boreholes for disposal, or whether, to be applicable, an adaptation of the requirements and criteria should be considered. Specific test criteria for deep borehole disposal are not available currently and plan-ning criteria need not be discussed here.

6.1 Geo-scientific require-ments and criteria for a mined repository applied to deep borehole disposal

The geo-scientific criteria for exclu-sion (large scale vertical movements, active faults, impact from mining, seismic and volcanic activity, age of groundwater) can be applied directly to the concept of borehole disposal since it is also disposal in a geological formation.

The minimum geo-scientifc criteria (permeability of formation, thickness of the CPRZ, depth of CPRZ, disposal area, period for proof) can be applied to the concept of borehole disposal in the same way as for a mined repo sitory, considering that the location of the CPRZ does not need to coincide with the location of the disposed waste.

Eleven weighting geo-scientifc requirements in three groups /KOM 16/ have to be discussed in more detail. The three groups were: quality

of containment and reliability of its evidence, validation of containment, and additional safety-relevant fea-tures (Table 2, Table 3 and Table 4).

All requirements and criteria are assessed using the generic concept for deep borehole disposal. At a later stage a site specific assessment is required. Due to the disposal in deep boreholes, some weighting criteria can be fullfilled favourably by sticking strictly to the definition of the CPRZ and host rock. Since in the generic concept of the DBD the CPRZ is not part of the host rock (disposal zone), but part of the overlying geological barriers, some criteria for safety relevant features should be applied to different rocks as well (No. 4 of Tab. 2, Tab. 3). A favourable weight-ing seems to be possible, but relative only to the underlying generic concept of DBD.

Two requirements with criteria (No. 2 and 3 of Tab. 4) have been assessed as not applicable to the concept of borehole disposal.

� Due to disposal depth, the temper-ature will be above 100 °C even without emplacement of heat generating radioactive waste. The

temperature of the rock and container will be in any case significantly above 100 °C. The heat generating waste will further increase the rock temperature, but the relatively small size of the container limits the rise in temper-ature. The suitability of the rocks for the rise in temperature has to be proven prior to disposal. No general temperature limit can be given here as it seems to be specific to the lithology.

� The present concept for containers and casing foresees use of steel and cast iron which will lead inevitably to some gas generation due to the expected presence of (ground-)-water. Even if the gas generation rate may be low, a future concept should minimize the use of steel and cast iron to minimize the potential generation of gas.

Two requirements about criteria (No. 4 and 5 of Tab. 4) are recom-mended for reassessment for applic-ability to DBD.

� The requirement to have a “high retention capability of CPRZ for radionuclides” could be extended to other rock formations available

No. Requirement Criteria DBD

1. No or slow transport with groundwater in the CPRZ

� Effective flow velocity less than 1 mm/a � Low available sources of groundwater � Low rate of diffusion.

Can be fulfilled depending on the definition and size of the CPRZ of the overlying rocks (salt / clay layer).

2. Favourable configu-ration of rock body, host rock and CPRZ

� Barrier efficiency (thickness and containment)

� Robustness and safety margins � Size of CPRZ � Clay rock: connection to water bearing strata and high hydraulic potential.

Can be fulfilled depending on the definition and size of the CPRZ of the overlying rocks (salt / clay layer) and host rock.

3. Good spatial characterisation

� Ascertainability (low variation and distribution of charateristic features of the CPRZ, low tectonic overprint)

� Transferability: large scale uniform or similar formation of rock of the CPRZ.

Can be fulfilled depending on the definition and size of the CPRZ of the overlying rocks (salt / clay layer). Tools to characterize host rock properties are available for greater depths.

4. Good predictability of the long-term stability of favourable conditions

Changes with time in � Thickness of the CPRZ � Size of the CPRZ � Permeability of rock formation.

Can be fulfilled depending on the definition and size of the CPRZ of the overlying rocks (salt / clay layer). The rock formation relevant for assessment of the permeability should be defined.

No. Requirement Criteria Borehole disposal

1. Favourable rock mechanics

� Low tendency to generation of secondary permeability by rock mechanics in host rock and CPRZ.

Can be fulfilled depending on the defini-tion and size of the CPRZ of the overlying rocks (salt / clay layer). The host rock formation relevant for assessment of the rock mechanics is in the disposal zone.

2. Low tendency to generation of groundwater flows in host rock and CPRZ

� Variability of permeability of rock formation.

� Reconstitution of cracks and secondary permeability by self-healing or closure of cracks.

Can be fulfilled depending on the defi ni-tion and size of the CPRZ of the overlying rocks (salt / clay layer). The host rock formation relevant for assessment of the groundwater flows is in the disposal zone.

| Tab. 2. Weighting group 1: quality of containment and reliability of its evidence.

| Tab. 3. Weighting group 2: validation of containment.



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below the CPRZ in the generic concept.

� The requirement for “favourable hydrochemistry“ has to be reas-sessed for the host rock to be useful in DBD since containment is not provided there.

6.2 Safety analysis and assessment

Safety analyses should be done to assess operational safety and the long-term safety of borehole disposal. A recent preliminary safety analysis on a generic concept showed good retention of radionuclides [ARN 13]. However, [ARN 13] did not consider all possibly relevant processes (gas flow, groundwater flow through faults, fractures and the EDZ). There-fore a more detailed operational and long-term safety analysis and assess-ment of the generic concept of DBD still has to be performed [BRA 16].

6.3 Retrievability / Recoverability

The [KOM 16] sets the general requirement for reversibility in the site selection process. On one hand, this concerns the site selection process itself, but on the other, it also has some technical implications which concern retrievability and recover-ability of disposed waste.

Retrievability is the planned tech-nical option for removing emplaced

radioactive waste containers from the repository facility during its opera-tional phase. This is also required by [BMU 10]. The containers and the borehole must allow retrieval until sealing and closure of the boreholes. It is for decision whether closure means a single borehole, a borehole field or all boreholes. At this stage, the un-derstanding is that disposal can be performed within a few years and the borehole is then sealed and closed. Based on experience in conventional drilling, it was assessed that retrieval of containers should be possible during a timeframe of at least 5 years after closure.

Recovery is the retrieval of radio-active waste from a final repository as an emergency measure after the oper-ational phase is over. This emergency may happen after the borehole is closed. The [BMU 10] requires a time period of 500 years for recovery of waste containers. The understanding is that the container and the casing should be designed to survive this time period without releasing radio-active contaminants or aerosols. Experience on recovery of lost objects in conventional drilling covers 100 years, but does not include recovery of corroded containers with HLRW.

Whereas, from the experts’ per-spective, retrievability seems to be manageable once the borehole and casing exists, recoverability from deep

boreholes needs some research and development to show if it is feasible.

6.4 HazardsHazardous incidents during the oper-ational phase might cause significant releases of radionuclides. Whereas volcanoes, earthquakes and other hazardous geological events should be excluded as far as possible with the site selection criteria, some opera-tional incidents nevertheless need to be assessed.

An incident might be a crack in the wall or break of a single container during disposal and subsequent the spent fuel or glass might contact the borehole fluid. Dissolution of the glass and an instant release fraction of spent fuel would contami-nate the borehole fluid. The amount of released radionuclide would depend on the contact time of fluid and waste and on the composition of the waste package. Measures for retrieval and repair should be provided and comply with radiation protection regulations.

A worst case, which should be unlikely, would be the loss of container within the subject of proctection and which cannot be retrieved for some reason. A release of radionuclides takes place in the long- or short-term and has to be assessed taking account of the geochemical and hydrological conditions.

No. Requirement Criteria Borehole disposal

1. Protective composition of overlying rocks

Protection of the CPRZ by � Coverage of the CPRZ with rock inhibiting groundwater flow � Distribution and thickness of such rocks � Distribution and thickness of erosion resistant rocks � No structural issues in coverage of rocks

A number of geological units with different but favourable characteristics, including additional redundant or diverse barriers, are possible in the generic concept due to depth of CPRZ's and disposal zone.

2. Good conditions to avoid or minimize gas generation

� The gas generation should be as low as possible for final disposal.

The present concept for container and casing foresees steel which will inevitably lead to some gas generation. Gas generation may be minimized or slowed down by choice of suitable borehole fluid or cementation of containers. A future concept may also minimize the use of steel or may foresee physical gas traps underneath the CPRZ. Gas generation cannot be completely avoided in the generic concept.

3. Good temperature compatibility

� A temperature of 100 °C is recommended for the surface of the canister.

The temperature in the disposal zone will be already higher than 100°C. It is therefore not a criterion for site selection and not applicable or useful for DBD.

4. High radionuclide retention capability of CPRZ

� High sorption capacity of rocks of CPRZ � High content of minerals with reactive surface in rock of CPRZ � High ion strength of the groundwater within the CPRZ � Size of pores in the CPRZ

Can be fulfilled depending on the definition and size of the CPRZ of the overlying rocks (salt / clay layer). Other rock formations could be relevant for assessment and should be defined.These criteria should also be re-evaluated for other rock formations and the host rock of the disposal zone.

5. Favourable hydrochemistry

The groundwater of the host rock / CPRZ shall � be in chemical equilibrium with the rocks � have pH of 7-8 � favourable redox conditions (anoxic-reducing) � have a low content of colloids and complexants � have a low concentration of carbonates

Can be fulfilled depending on the definition and size of the CPRZ of the overlying rocks (salt / clay layer) which is not part of the host rock. Other rock formations could be relevant for assessment and should be defined.These criteria should be re-evaluated also for other rock formations and the host rock of the disposal zone.

| Tab. 4. Weighting group 3: further safety relevant features.



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The long term safety analysis should consider that the containers currently proposed for deep borehole disposal are not corrosion resistant in the long-term with respect to groundwater. Even if corrosion may take place slow-ly, some hydrogen gas generation and other corrosion processes will occur. This may increase the pressure within the sealed borehole and may impact the trans portation of released radio-nuclides. Heat generation and high temperatures in the disposal zone will speed up chemical processes, which cannot be modelled in detail due to a lack of thermodynamic data.

The total mass of fissile radio-nuclides in a deep borehole will be above the critical mass. Therefore a possible critical excursion and safety measures have to be assessed. Although preliminary safety analysis excluded critical excursions due to the low solubility and mobility of U(IV), an assessment and optimisation should be performed.

7 Research and development

Boreholes with diameters of 0.75 m at 3,500 m depth are still beyond today's standard technology, but are con sidered feasible. Based on this, a concept for disposal of radioactive waste in deep boreholes is drafted. Further development and demonstra-tion of dedicated borehole technology is necessary to demonstrate routine technical feasibility.

The disposal with recoverability according to [BMU 10] also necessi-tates research and development into the long-term behaviour of the container and casing.

The operational phase for disposal of radioactive waste in deep boreholes requires investigations in detail about its safety and radiation protection. There is a need for development testing and demonstration. This includes the feasibilty of retrieval before closure.

If the current proposal of the Commission about recoverability for 500 years remains a prerequisite, it would emphasise the need for research and development on con-tainers and technology.

8 Summary and conclusionUsing deep boreholes for disposal (DBD) of high-level radioactive waste (HLRW) can have advantages for long-term safety due to an ample distance between the HLRW and the biosphere (subject of protection) and may take advantage of multiple

geologic barriers as safety features. The great depth and short disposal operation efficiently impedes prolifer-ation. Finally, aside from site selection process, there may be a time related benefit for technical implementation and for costs of implementation.

A generic concept for DBD of HLRW has been developed, applying con-tainment providing rock zones (CPRZ). Although further technical develop-ments are required for HLRW disposal in deep boreholes to address larger than usual diameter and depth of boreholes, DBD seems to be feasible as an alter-native option for geological disposal of radioactive waste. Further research and development with a feasibility demon-stration is necessary. Operational and long-term safety analyses and assess-ments have to be performed.

On one hand, a major is the requirement for possible recovery of waste for 500 years after closure. On the other hand if disposal is intended to be a permanent and the most safe solution, recovery might not be the main focus of the decision when high-est possible safety is desired. During policy decision-making, if there are clear advantages for long-term safety with DBD, these might outweigh the disadvantage with recovery.

The Commission asked initially for borehole diameters of 1 m in 5,000 m based on plans for DBD in the USA [NWTRB 16] which set the initial framework for the generic concept developed, presented and discussed here. Based on the current state of knowledge, it was assessed that this diameter/depth cannot be safely operated. Reducing the depth and therefore necessary borehole and con-tainer diameters for disposal lowers the technical challenges without jeopardizing the potential safety benefits of DBD. The favourable depth for DBD ranges from 1,500 to 3,500 m.

DBD’s advantages in safety, speed, as well as cost were discussed, and that DBD might be seen as an alter-native option for geological disposal in Germany. To have this option avail-able as a proven technology during a policy decision, it seems very sensible to follow up the DBD concept with installation of a real scale demonstra-tion along with a detailed safety analysis. DBD could then provide technical redundancy – if required – in case the siting or implementation of a mined repository fails or cannot be pursued any longer for some reason.

Having this in mind, DBD may have a chance in Germany in the long run.

AcknowledgementThe authors thank Horst Geckeis, KIT, for his critical comments, which clarified the paper’s contribution to this important subject.

References

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Decommissioning and Waste ManagementThermal Margin Comparison Between DAM and Simple Model ı Jeonghun Cha and Daesik Yook

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Thermal Margin Comparison Between DAM and Simple ModelJeonghun Cha and Daesik Yook

Introduction Nuclear industry in Korea prefers dry storage system to store spent nuclear fuel (NSF) as an interim storage and demand of adopting relevant technologies is increasing. Safety of dry storage system is to keep integrity of NSF cladding. And the maximum temperature of cladding is the most important factor to keep its integrity in field of thermal analysis. According to Relevant guideline called ISA-11 (Interim Staff Guideline, Cladding Consideration for the Transportation and Storage of Spent Fuel) in USA, the maximum temperature of PWR fuel cladding for normal operating condition and abnormal condition, are 673 K and 843 K, respectively. And USA’s nuclear industry shows thermal safety using CFD, according to ISG-21 (Use of Computational Modeling Software). Corresponding to increasing of computer hardware performance, the industry develop more complex analysis model and expect increasing thermal margin.

Numerical models and assumptions The Korea standard PWR spent fuel, which was proposed by KAERI (Korea Atomic Energy Research Institute), is considered in this study. It is assumed that the PWR spent fuel has cooled for

10 years, arrayed 16 by 16 and 938.8 W decay heat (each rod has 3.978 W).

The DAM described 1/4 storage canister refer to a commercial canister. And it is assumed that heat transfer between each channel is ignored, helium gas is filled inner canister and

operation pressure is 2 atm. Its numerical model consists of about 7.4 M nodes. Figure 1 shows the numerical model.

Five different canister surface temperatures are set as an input condition and at each case, mass flow

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[JUH 98] Juhlin, C., Wallroth, T., Smellie, J., Eliasson, T., Ljunggren, C., Leijon, B., Beswick, A.J.: The Very Deep Hole Concept – Geoscientific appraisal of conditions at great depth. Technical Report. Ed.: Svensk Kärnbränslehantering, A.B., TR-98-05: Stockholm, Sweden, 1998.

[KOM 16] Kommission Lagerung hoch radioaktiver Abfallstoffe: Abschlussbericht der Kommission Lagerung hoch radioaktiver Abfallstoffe. K-Drs. 268, 683 pp.: Berlin, 30. August 2016.

[KRE 09] Kretzschmar, H.J., Schmitz, S., Schreiner, W., Lendel, U.: Salz stopfen-Versatz von CO2-Speicherbohrungen. Erdöl, Erdgas, Kohle, Vol. 125, No. 11, p. 440, 2009.

[KWI 08] Kwiatek, G., Bohnhoff, M., Dresen, G., Schulze, A., Schulte, T., Zimmermann, G., Huenges, E.: Microseismic event Analysis in conjunction with stimulation treatments at the geothermal resarch well GTGRSK4/05 in Groß Schönebeck/Germany. 2008.

[NWTRB 16] Nuclear Waste Technical Review Board (NWTRB): Technical Evaluation of the U.S. Department of Energy Deep Borehole Disposal Research and Development Program. A Report to the U.S. Congress and the Secretary of Energy. Ed.: Nuclear Waste Technical Review Board, 70 pp.: Arlington, Virginia, 2016.

[TEN 00] Tenzer, H., Schanz, U., Homeier, G. (Eds.): HDR research programme and results of drill hole Urach 3 to depth of 4440 m – the key for realisation of a hdr programme in southern Germany and northern Switzerland. Proceedings World Geothermal Congress 2000, 3927-3932, 2000.

[TIS 06] Tischner, T., Pfender, M., Teza, D.: Hot Dry Rock Projekt Soultz: Erste Phase der Erstellung einer wissenschaftlichen Pilotanlage. Abschlussbericht zum BMU-Vorhaben. No. 0327097, 85 pp., 2006.

Authors Guido Bracke GRS gGmbH Schwertnergasse 1 50667 Köln, Germany Frank Charlier RWTH Aachen Nukleare Entsorgung und Techniktransfer (NET) 52062 Aachen, Germany Axel Liebscher Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences Telegrafenberg 14473 Potsdam, Germany Frank Schilling KIT Karlsruher Institute of Technology – Technical University Karlsruhe Institute for Applied Geosciences Adenauerring 20b 76131 Karlsruhe, Germany Thomas Röckel Piewak & Partner Jean-Paul-Str. 30 95444 Bayreuth, Germany

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Does Deep Borehole Disposal of HLRW has a …...atw Vol. 62 (2017) | Issue 1 ı January Decommissioning and Waste Management Does Deep Borehole Disposal of HLRW has a Chance in Germany?